Abstract: Based on particle-in-cell method,a two-dimensional numerical model has been developed to investigate the ion beam extraction phenomena through the optics system of ion thruster.According to the calculation results,the distribution of electric potential between screen grid and accelerator grid,the ion and electron spatial distribution were obtained for various accelerator voltages and accelerator grid radii.The simulation results show good agreement with the experimental data.The results indicate that the accelerator voltage and the accelerator radius have great influences on electron backstreaming.Electron backstreaming happens when the absolute value of accelerator voltage is more negative.The electrons move downstream with the decrease of accelerator voltage.Moreover,electron backstreaming occurs with the enlargement of accelerator radius.Under moderate accelerator voltage and accelerator radius,the electron backstreaming will not happen,and the ion thruster will reach its better performance.